Fixed links in download page (#2654)
Fixed links in download page (#2654)

### Motivation

The existing releases were moved from `incubator/pulsar` to `pulsar` folder in mirrors. There's no need to differentiate between incubator and later releases in terms of paths.
massakam/pulsar,ArvinDevel/incubator-pulsar,ArvinDevel/incubator-pulsar,yahoo/pulsar,jai1/pulsar,nkurihar/pulsar,nkurihar/pulsar,jai1/pulsar,yahoo/pulsar,ArvinDevel/incubator-pulsar,merlimat/pulsar,merlimat/pulsar,merlimat/pulsar,ArvinDevel/incubator-pulsar,jai1/pulsar,massakam/pulsar,yahoo/pulsar,merlimat/pulsar,merlimat/pulsar,jai1/pulsar,jai1/pulsar,ArvinDevel/incubator-pulsar,jai1/pulsar,yahoo/pulsar,nkurihar/pulsar,massakam/pulsar,merlimat/pulsar,nkurihar/pulsar,massakam/pulsar,massakam/pulsar,massakam/pulsar,ArvinDevel/incubator-pulsar,nkurihar/pulsar,yahoo/pulsar,nkurihar/pulsar,ArvinDevel/incubator-pulsar,jai1/pulsar,ArvinDevel/incubator-pulsar,yahoo/pulsar,nkurihar/pulsar,jai1/pulsar,jai1/pulsar,nkurihar/pulsar,nkurihar/pulsar